1. Field of the Invention
This invention relates to a method of calibrating a time-interleaved analogue-to-digital sampler and to a method of performing analogue-to-digital conversion with a sampler so calibrated. The invention also extends to a calibration apparatus and to a channel equaliser.
2. Discussion of Prior Art
A commonly employed method of building high-speed samplers is to time-interleave multiple channels, each channel having its own analogue-to-digital converter (ADC). Each ADC takes it in turn to sample the analogue signal according to a sampling sequence, thereby producing its own digital signal that follows the shape of the analogue signal but at the sampling rate of the ADC. The digital signals from the ADCs can then be interleaved according to their sampling sequence to produce an output signal that follows the shape of the analogue signal, but at a faster effective sampling rate (for N channels, the sampling rate is N times the sampling rate of each ADC).
A problem with this approach is that the channels are not identical and sampling is not performed at exactly equally-spaced intervals, thereby degrading the performance of the sampler. This degradation can most simply be seen in the frequency spectrum of the time-interleaved digital output signal produced in response to a tone input: such a spectrum will contain spurious signals at specific frequencies in addition to the wanted signal at the frequency of the tone input. It is known that if a tone is injected into an N-channel time-interleaved sampler, mismatches between the N channels will lead to the generation of N frequency spikes in the range from zero to half the overall sampling frequency of the sampler (the N spikes comprising N−1 spurious spikes plus the wanted signal at the frequency of the tone input). Hence, signals are observed in the frequency spectrum at N related frequencies. The N frequencies are mixed by mismatches without affecting output across the rest of the frequency spectrum. Moreover, if a tone is input at any one of the N related frequencies, only those same N frequencies are affected.